In the semiconductor industry, copper interconnects are increasingly being used as an interconnect material rather than aluminum. The superior electrical conductivity of copper over aluminum may result in higher speed interconnections of greater current carrying capability. Currently, copper interconnects are formed using a so-called “damascene” or “dual-damascene” fabrication process. Briefly, a damascene metallization process forms interconnects by the deposition of conducting metals in recesses formed on a semiconductor wafer surface. Typically, semiconductor devices (e.g., integrated circuits) are formed on a semiconductor substrate. These substrates are generally covered with an oxide layer. Material may be removed from selected regions of the oxide layer creating openings referred to as in-laid regions within the substrate surface. These in-laid regions correspond to a circuit interconnect pattern forming the conductor wiring of the device.
Once the in-laid pattern has been formed within the oxide layer, a thin barrier layer may be fabricated that evenly blankets the patterned oxide layer. This barrier layer may composed of, but is not limited to, titanium nitride, tantalum nitride, or tungsten nitride. After the barrier layer is formed, a seed layer of a conductive metal, preferably comprising copper, is deposited. The seed layer of conductive metal or copper forms the foundation for the bulk deposition of copper by a variety of deposition techniques including, but not limited to, physical sputtering, chemical vapor deposition (CVD) or electroplating. After the bulk copper has been deposited, excess copper may be removed using, for example, by chemical-mechanical polishing (CMP). The surface is then cleaned and sealed with a sealing layer. Further processing may then be performed.
An ongoing difficulty in the manufacture of semiconductor devices is the increasing miniaturization of the devices. In some devices, the smallest lines and in-laid regions may be less than 0.2 microns wide and over 1 micron deep yet it is becoming increasingly critical that the seed layer be continuous, smooth, and conformal along the sidewalls and bottom of these in-laid regions. Thus, there is a growing need for processes capable of growing extremely thin and highly conformal metal films or seed layers inside these microscopic lines and in-laid regions without leaving any voids that could lead to electrical failures in the finished product. One particularly attractive technique for growing such ultra conformal metal films is Atomic Layer Deposition (“ALD”). It is envisioned that this technique may be particularly useful in the formation of barrier and/or seed layers in semiconductor devices having high aspect ratios, i.e., deep trenches having narrow trench widths. This technique has been particularly useful for forming thin, conformal layers of a variety of formulations such as titanium nitride, aluminum oxide, and copper.
In a typical ALD process, one or more reagents, referred to herein as precursors, are chemisorbed onto a substrate surface to form a monolayer of precursor that is approximately one molecule thick. A second reagent may be introduced to chemically react with the first chemisorbed layer to grow the desired film on the substrate surface. The first reagent can then be reintroduced and the cycle repeated. Reagents or precursors that are best suited for an ALD process are readily volatile, have high chemical reactivity, form readily volatile by-products, and, at the molecular level, are dense in the element to be deposited (reacted) onto the substrate surface. After sufficient process cycles of monolayer formation has occurred, the process can be terminated to yield the final film. Alternatively, the film may be created by reacting the monolayers by thermal processing or by chemical reduction via a reducing agent.
The references, Higashi et al., “Sequential Surface Chemical Reaction Limited Growth of High Quality Al2O3 Dielectrics” Applied Physics Letter, Vol 55, No. 19 (1989), pp. 1963-65 and S. M. George, et al., 3rd Internal Symposium on Atomic Layer Epitaxy and Related Surface Processes, describe ALD processes for the growth of aluminum oxide from trimethylaluminum and water vapor reagents. In this process, it is believed that the trimethylaluminum vapor may react strongly with a surface monolayer of hydroxyl groups resulting from a prior exposure to water vapor. In the ensuing reaction, it is believed that aluminum-oxygen bonds are formed and methane gas is released as a result of the methyl groups within the trimethylaluminum reagent abstracting hydrogen from the hydroxyl groups on the surface. Residual methyl groups on the aluminum atoms are perceived to be bonded to the surface by the oxygen present are then reacted away to provide additional methane and surface Al—OH groups when a subsequent pulse of water vapor is introduced. These surface Al—OH groups may be reacted with additional trimethylaluminum to provide further Al—OH groups. Thus, a network of aluminum oxide is grown as these alternating cycles of trimethylaluminum and water vapor pulses are repeated. Excess reagent is readily purged away from the surface after each pulse because there are only a finite number of surface reactive sites available to form bonds. Consequently, monolayer coverage of the surface may be readily achieved.
The semiconductor fabrication industry has employed a variety of ALD processes for growing copper or copper containing films, either on diffusion barrier and/or adhesion promotion materials or directly upon semiconductor materials, using a variety of different copper precursors and reducing agents. For example, the reference, Per Martensson, et al., “Atomic Layer Epitaxy of Copper”, J. Electrochem. Soc., Vol. 145, No. 8, August 1998, pp. 2926-31 (“Per Martensson I”), describes an ALD process for growing copper films using hydrogen gas as the reducing agent to reduce the adsorbed monolayers of the copper β-diketonates, Cu(II)-bis(2,2,6,6-tetramethyl-3-5-heptadionate) (“Cu+2(thd)”). The Cu+2(thd) copper precursor, however, is a relatively large and bulky molecule. Because of this, the amount of copper in an adsorbed monolayer of Cu+2(thd) copper is relatively low. Hence, more process cycles may need to be performed to have sufficient deposition of copper.
The reference, Raj Solanki, et al., “Atomic Layer Deposition of Copper Seed Layers”, Electrochemical and Solid-State Letters, Vol. 3 (10) (2000), pp. 497-480 (“Raj Solanki”), describes an ALD process for growing copper films using ethanol, methanol or formalin, as the reducing agents to reduce the adsorbed monolayers of the copper bis(β-diketonate), Cu(II) bis(1,1,1,5,5,5-hexafluoroacetylacetonate hydrate) (“Cu+2(hfac)2). One challenge with this approach is that the copper precursor used may be unstable with respect to its loss of water of hydration. This could lead to problems of inconsistency in delivering the precursor to the substrate surface.
The references, Per Martensson, et al., “Atomic Layer Epitaxy of Copper on Tantalum”, Chem. Vap. Deposition, Vol. 3, No. 1 (1997), pp. 45-50 (“Per Martensson II”) and published U.S. Pat. Application No. 2002/0106846, describe ALD processes for growing copper seed layers using copper (+1) chloride as the copper source and hydrogen gas or triethyl boron (TEB), respectively, as the reducing agents to reduce adsorbed monolayers of copper chloride. Copper chloride may present difficulties in vapor delivery due to its relatively low volatility and high melting point (430°).
Published U.S. Patent Application US 2002/0004293 A1 describes an ALD process for growing a copper oxide film. The copper oxide film is grown in an ALD process by alternating the Cu+2(thd) copper precursor with ozone. The copper oxide layer can then be reduced using a variety of reagents. Like Per Martensson I, this method may suffer from using the bulky Cu+2(thd) copper precursor. Further, the ozone precursor is a harsh oxidizing agent potentially capable of irreversibly damaging the substrate by oxidizing the underlying barrier layer.
In addition to the problems cited above, these prior art methods for forming copper ALD films also suffer from relatively weak absorption between the copper precursor and underlying substrate. Unlike the ALD process for growing aluminum oxide layers wherein the aluminum from the trimethyaluminum precursor forms a strong bond with oxygen because of the hydroxyl groups on the substrate surface, the prior art ALD processes for growing copper layers may have relatively weaker chemisorption of the copper precursor onto the substrate surface because the metal or metal oxide substrate surface lacks strongly coordinating sites or chemical reactivity to bond the precursor to the surface. While not wishing to be bound by theory, weaker absorption can result in intermittent and patchy coverage in that some areas of the substrate may have incomplete coverage while other areas may have more than one monolayer of coverage ultimately resulting in an uneven and rough copper film. Because there is no strong chemical bond or relative attraction between the underlying surface and the copper precursor, achieving uniform monolayer coverage prior to reduction of the copper precursor presents more of a challenge. Subsequently, the pressure of the reactor, the flow of the precursor, the choice of carrier gas, etc., may have to be carefully controlled in order to achieve uniform monolayer coverage. As a result, the processing window for these parameters becomes narrower and the possibility of reduced process yield and diminished throughput increases.
Accordingly, there is a need in the art to provide a method, particularly an ALD-based method for forming a thin metal film, such as a copper or copper containing film film, onto the surface of a substrate which results in chemisorbed layers having excellent conformality. There is an additional need to provide a metal, particularly a copper or copper containing, film with improved adhesion to the underlying substrate material or underlying diffusion barrier and/or adhesion promotion material.
All references disclosed herein are incorporated by reference in its entirety.